Modern aircraft secondary power systems have a gas turbine powered auxiliary power unit (APU) on board to provide electrical, hydraulic and pneumatic power for operating systems aboard the aircraft when the propulsion engines are shut down as well as to provide power for starting the propulsion engines. Typically, pneumatic power, provided by a load compressor within the APU, is used to start the propulsion engines.
An electrical start system that comprises a starter or starter generator initiates ignition of the APU. The start system must generate enough torque to overcome drag or resisting torque of the APU and to accelerate the APU to achieve light off and self-sustaining rotational speed. The drag torque of an APU increases as the temperature of the APU decreases. This is due to several factors, one of which is increased drag due to residual oil left in the APU.
Typically, an APU uses a start system with a DC brushed starter motor that must be sized to provide enough torque to start the APU during the most severe environmental conditions when APU drag torque levels peak. Such a start system must then draw a substantial amount of current under all environmental conditions even though such current levels are only needed to overcome peak drag torque levels encountered under severe environmental conditions.
Because the start system generates so much torque, it may cause the APU to accelerate too quickly under average conditions, possibly causing the APU to pass through its light-off window before it has a chance to start, thereby causing a failed start. It also results in high inrush currents that may require aircraft storage batteries and electrical cables that supply the start system in order to avoid large electrical potential drops.
One prior art approach has been to control acceleration of a start system to a set of pre-determined acceleration values for the starting, light-off and post-ignition phases of APU operation. Adjustment of starting system torque is based upon comparison of the actual acceleration values to the pre-determined acceleration values. See U.S. Pat. No. 5,428,275 to Carr et al. Another prior art approach has been to control torque of a start system to follow a predetermined acceleration schedule that is selected based upon the speed, oil temperature and inlet temperature of the APU. See U.S. Pat. No. 6,035,626 to Wahl et al. Although these approaches can be effective, a simpler approach to controlling start system torque is desirable.